1. Field of the Invention
The present invention relates to a joint structure for jointing a diamond body to a metallic body, and a pressure detector having a strain gauge made of diamond semiconductor, which makes use of this joint structure.
2. Description of the Prior Art
Conventionally, Si semiconductors have been used in many cases. However, the band gap of Si is as small as about 1.1 eV. This results in the semiconductor characteristic thereof disappearing when the ambient temperature is over about 150.degree. C.
In contrast, the band gap of diamond is as large as about 5.5 eV as compared with that of Si. Therefore, diamond semiconductor is expected to be durable in a high temperature atmosphere.
Furthermore, diamond monocrystal is excellent in its electrically insulating characteristic and very large in its thermal conductivity. Therefore, diamond monocrystal is expected to exhibit excellent performance as a semiconductor base plate.
Recently, it has been strongly desired to directly detect the explosion stroke of each cylinder in the electronic fuel injection control(EFI) of a vehicle engine. And to this end, such a pressure detector as is capable of directly detecting the pressure within each cylinder has been demanded.
The pressure detector is generally provided with a pressure chamber for introducing a pressure to be measured into a housing projecting within an atmosphere to be measured. One portion of a wall defining the pressure chamber is composed of a diaphragm which deforms in accordance with the change of the pressure to be measured. To to this diaphragm is secured a strain gauge which generates an output signal corresponding to the strain of the diaphragm.
A semiconductor strain gauge has been noted as the strain gauge one to be considered for use in such an instance because of very good sensitivity. However, the usable temperature range of the Si semiconductor is low as described above. Therefore, it has heretofore been impossible to detect the pressure within engine cylinders of which the temperature exceeds 500.degree. to 600.degree. C.
Under the above circumstances, it can be proposed to use diamond of which the usable temperature is high, as the strain gauge. However, when the diamond monocrystal or semiconductor is installed in the pressure detector, there occurs a problem in joining it to the metallic body. Namely, the coefficient of thermal expansion of diamond is as small as about 2.3.times.10.sup.-6 /.degree.C. at 100.degree. C. When the diamond is joined to a metal such as stainless steel (coefficient of thermal expansion is about 15.times.10.sup.-6 /.degree.C. at 100.degree. C.), damages sometimes occur in the joint portions therebetween due to the change of temperature of an ambient atmosphere because of a large difference in coefficient of thermal expansion.